1. Field of the Invention
The invention relates to a method for screening state variables, in particular temperatures, in an oil-cooled transformer.
2. Discussion of Background Information
Transformers are electrotechnical devices which have today already reached a very high level of technical and technological sophistication. Nevertheless, further developments are still possible and necessary, but involve a correspondingly high level of expenditure and risk, including from a commercial viewpoint.
In the currently evolving situation with the free exchange of energy over large distances, the question concerning the extent to which a transformer can be operated with overload without a significant loss in terms of service life is gaining increasingly in importance.
To be regarded as the current state of the art in this respect are the documents U.S. Pat. No. 4,654,804 and FR-A-2 526 599.
The first-mentioned document discloses a method for monitoring and analyzing transformer parameters, such as for example temperatures, winding currents, oil pressure, etc. All the parameters to be monitored must in this case be picked up by use of sensors and they are then compared with predetermined limit values. For this purpose, limit values which do not take into consideration external conditions surrounding the transformer, such as for example the ambient temperature, must be fixed in advance. Since the analysis of all the transformer parameters is based here on a simple comparison, this method also cannot be used for a simulation of specific situations.
The second document relates, inter alia, to an air-cooled transformer, that is to say to a thermopneumatic system and not to a thermohydraulic system, as is present in an oil-cooled transformer.
An aspect of the invention is consequently to provide a method which supplies information on important temperatures, for example the hotspot and hot-oil temperatures, and can additionally be used for forecasting, simulation and analysis.
This aspect is achieved by input variables, in particular the voltages at the transformer terminals, the currents in the windings and the ambient temperature, being measured, and the status of cooling units, which are fans, pumps etc., and possibly the position of a switch, such as for example a stepping switch or the like being established, by these input variables and the status of the cooling units and the switch position being fed to a thermohydraulic model and by state variables being calculated in the thermohydraulic model with auxiliary variables, which are for example losses in the transformer, parameters for the heat transfer, flow resistances and the flow rates in the oil circuit, and a hydraulic network of the oil circuit, which has branches and nodes, these state variables preferably being the average temperatures and the hotspot temperatures in the loss-producing parts of the transformer and the average oil temperatures in branches and in nodes of the hydraulic network of the oil circuit, and by the auxiliary variables being adapted appropriately to the current value of the state variables when there is a change in the input variables and/or the status of the cooling units and/or of the switch position, and the change over time of the state variables subsequently being calculated and new state variables being calculated with these changes. With this method, it is consequently possible for the first time to ascertain the operating temperatures and also the critical temperatures as well as their changes in parts of the transformer without temperature sensors, whereby the optimum operation of the transformer is ensured. With the thermohydraulic model of the transformer cooled with oil, the behavior of the temperatures in the core, the windings and in the oil is ascertained both in the case of steady-state processes and in the case of transient processes. The suffix xe2x80x9chydraulicxe2x80x9d indicates that the hydraulic behavior of the oil is also described by the model.
According to an aspect of the present invention, a method is provided wherein the rate of change of the state variables and the state variables are calculated with differential equations. This aspect is advantageous, since it is the way leading most directly to the aim of ascertaining the rate of change of the state variables.
According to another aspect of the present invention, the measured voltages are used to ascertain the relationship between these voltages and the magnetic flux in the individual parts of the core of the transformer by means of an assignment matrix, and wherein the idling losses are subsequently ascertained in dependence on this magnetic flux by a characteristic curve defined by parameters. The aforementioned aspect makes it possible to calculate on a digital computer the idling losses in the individual parts of the core directly from the terminal voltages.
In yet another aspect of the invention, the transformer-internal winding branches are defined, linked with the measured currents, preferably the terminal currents, by the use of a current assignment matrix. All the currents are represented as a vector with two components because of the necessity to take the phase position into consideration. The amount of the current with which the ohmic losses are calculated via the appropriate resistance is ascertained for each internal winding branch. The distribution of the magnetic leakage flux is ascertained from the distribution of the currents between the individual windings with matrices which are defined both for axial components and for radial components. From the matrices, the eddy-current losses in the windings and the inactive parts of the transformer are ascertained. Also, the relationship between the currents in winding branches and the magnetic leakage flux relevant for the loss-producing branches is ascertained by a matrix, and the eddy-current losses are calculated with factors which are ascertained from the winding geometry. According to the implementational variant discussed above, all the increases in temperature of parts in the transformer caused by the measure currents are ascertained.
In another aspect of the present invention, the matrices and the relevant assignment matrix and ohmic winding resistances are dependent on the switch position. According to a further aspect of the present invention, an initializing operation is carried out with every change, of the switch position, the voltagexe2x80x94magnetic flux assignment matrix, the currentsxe2x80x94internal winding currents matrix and the ohmic winding resistances being checked and changed if appropriate. The aspects above take into consideration the influence of the switch position on the distribution of the main magnetic flux the distribution of the internal currents and also on the values of the ohmic resistances of individual winding branches.
In another aspect of the present invention, the status of the cooling units is included in at least one of the parameters of the heat transfer between the oil and the ambience, and the corresponding relationship between the fans and the cooling branches is expressed in an assignment matrix. According to this aspect, the influence of the operating state of the fans on the cooling is described.
According to a further aspect of the present invention, the average temperatures in loss-producing parts of the transformer are calculated with the differential equation:             ⅆ              ⅆ        t              ⁢    tqm    =                    V        tot            -      wim        cqg  
wherein tqm is the average temperature of a loss-producing part of the transformer, Vtot is the total losses of the loss-producing part of the transformer, cqg is the thermal capacity of the loss-producing part of the transformer, and wim is the power loss dissipated from the loss-producing part of the transformer to the oil flowing past. This configuration is advantageous, since with this differential equation the average temperatures in loss-producing parts are ascertained in a simple way.
In another aspect of the present invention, the power loss wim dissipated to the oil flowing past is calculated with the formula:   wim  =      V0g    ·                  (                  g          g0                )            xg      
wherein V0g are reference losses, g is the difference between the average temperature of the loss-producing part of the transformer and the average temperature of the oil in contact, referred to as the average jump, g0 is the average jump in the steady state and with the reference losses and xg is the exponent for heat transfer between the loss-producing part of the transformer and the oil, in dependence on the oil temperature and oil velocity. In the case of the aforementioned aspect, it is of advantage that the power loss dissipated to the oil flowing past, this loss being entered into the differential equation for the average temperatures, is ascertained with only a few calculating operations. This power loss is always for that part for which the average temperature is being calculated with the differential equation.
In another aspect of the invention, the hotspot temperatures in loss-producing parts of the transformer are calculated with the differential equation:             ⅆ              ⅆ        t              ⁢    tqh    =                    V        hot            -      wih        cqh  
wherein tqh is the hotspot temperature of a loss-producing part of the transformer, Vhot is the total losses of the loss-producing part of the transformer multiplied by a xe2x80x9chotspot factorxe2x80x9d, cqh is the thermal capacity of the loss-producing part of the transformer converted to the conditions at the location of maximum temperature and wih is the power loss dissipated by the hottest location of the loss-producing part of the transformer to the oil flowing past. This aspect is also of advantage, since the differential equation for the hotspot temperatures in the loss-producing parts has the same calculating operations as that for the average temperatures; if these equations are resolved on a digital computer, the same program steps can be used. The differences at the hotspot in comparison with the average values with regard to generated-power loss, heat transfer and thermal capacity can nevertheless be exactly predetermined via the corresponding parameters.
According to a still further aspect of the present invention, the power loss wih dissipated by a hotspot to the oil flowing past is calculated with the formula:   wih  =      V0h    ·                  (                  h          h0                )            xh      
wherein V0h are reference losses for the hotspot, h is the difference between the maximum temperature of the loss-producing part of the transformer and the oil temperature at the hottest location of the part of the transformer, referred to as the hotspot jump, h0 is the hotspot jump in the steady state and with the reference losses for the hotspot and xh is the exponent for heat transfer between the loss-producing part of the transformer and the oil, in dependence on the oil temperature and oil velocity. The configuration discussed above is also advantageous, because the power loss is always ascertained for that part for which the hotspot temperature is currently being established with the differential equation.
Further aspects of the invention include, wherein the average oil temperatures in branches of the hydraulic network of the oil circuit is calculated with the differential equation             ⅆ              ⅆ        t              ⁢    tom    =            wim      -      wam      -              phz        ·        D                    coel      z      
wherein tom is the average oil temperature in an oil flow branch, wim is the power loss dissipated by the loss-producing part of the transformer to the oil flowing past, warn is the power loss dissipated by a branch to the ambience, phz is the oil flow through the branch, expressed in transposed thermal output per degree of difference in temperature between the oil temperature at the beginning of the branch and the end of the branch, is the difference in temperature between the oil temperature at the beginning of the branch and the end of the branch and coelz, is the thermal capacity of the oil in the flow branch. In the case of the embodiment above, the average oil temperatures in branches of the hydraulic network of the oil circuit are calculated with a simple differential equation.
According to other aspects of the present invention, the thermal power wam dissipated by a branch to the ambience is calculated with the formula:   wam  =            wam0      ·                        (                                    υ              xc2x0                                                      υ                xc2x0                            0                                )                                      x            ⁢                          υ              xc2x0                                ⁢                      xe2x80x83                              ·      fup        -    VK    -    sun  
wherein wam is the reference value of the dissipated thermal power,  is the difference in temperature between the average oil temperature in the branch and the ambient temperature, 0 is the reference value for the difference in temperature  for which the value wam0 is defined, x is the exponent for the heat transfer between the oil and the ambience, in dependence on the type cooling, fup is a factor for the influence of the ambient temperature and if appropriate the air pressure, VK is the leakage power loss in the tank, and if appropriate of a cooler branch which represents a surface of the tank, and sun is the power of the solar irradiation. According to this advantageous development, the thermal power dissipated to the ambience from that branch for which the average oil temperature is subsequently established with the differential equation is ascertained.
According to another aspect of the present invention, the average oil temperatures in nodes of the hydraulic network of the oil circuit is calculated with the differential equation:             ⅆ              ⅆ        t              ⁢    tok    =                    ∑                  i          =          1                          n          z                    ⁢              xe2x80x83            ⁢              phz        ·                  T          eff                            coel      K      
wherein tok is the average oil temperature in a node, nz is the number of branches which enter at this node, phz is the oil flow through one of the entry branches, Teff is the temperature of the oil at the end of the branch connected to the node, which is essentially dependent on the direction of flow, and coelk is the thermal capacity of the oil in this node. The development discussed above is also of advantage, this providing that the average oil temperature in a node is ascertained, with inclusion of the oil flow, using that oil flow which was already used in the calculation of the average oil temperature through one of the entering branches.
According to another aspect of the present invention, the differential equations are resolved by a known numerical method, for example the method, according to Runge-Kutta. This development is absolutely necessary because of the frequently occurring nonlinear relationships and the method of calculation of the individual temperatures, which is partly dependent on the direction of flow of the oil.
In yet another aspect of the present invention the branches and also the nodes are assigned a certain oil volume, which together corresponds to the total oil volume. A design as described above, in which checking of the hydraulic network is carried out, is also advantageous.
In another aspect of the present invention, when resolving the differential equations for the average oil temperatures (tom) in the branches and in the nodes of the hydraulic network of the oil circuit, the state of flow is continuously also calculated by a system of equations for the pressure drops in all the branches of the entire hydraulic network. In this embodiment, the state of flow, described by the branch variables phz, which can change in the hydraulic network on account of transient processes, is continuously calculated at the same time.
According to a further aspect of the present invention, for each branch in the hydraulic network, the vector for a virtual driving pressure difference f is ascertained:
f=gxc2x7xcfx81xc2x7xcex2xc2x7xcex94Hxc2x7T+pd
wherein f is the driving pressure difference, g is the acceleration due to gravity, xcfx81 is the density of the oil, xcex2 is the coefficient of expansion of the oil, xcex94H is the difference in height between the starting point and end point of the branch is the average temperature of the oil in the branch and pd is, if there is one, the pressure difference caused by a pump in the branch, and wherein the vector xcfx86 of the oil flow in the individual branches is calculated with the matrix equation xcfx86=TTRTTxc2x7f, TTRTT being a matrix which contains the information on the hydraulic resistances in all the branches and on the structure of the hydraulic network. This aspect, with which the driving pressure differences in the flow branches, and from them the flow vectors in individual branches, are ascertained, is also of advantage. The flow of the oil in the hydraulic network is determined by the hydraulic resistances of the individual flow branches, by the distribution of the temperatures of the oil and, if there are any, by the pressure and flow characteristic of pumps.
In another aspect of the present invention, the vectors xcfx86 of the oil flow are ascertained in an iterative process, this process being continued until the initial value and result coincide with adequate accuracy. This development is necessary, since the hydraulic resistances are non-linear.
According to a still further aspect of the present invention, a continuous adaptation of parameters is carried out with two auxiliary variables, one auxiliary variable taking into consideration the dependence of the losses on the temperature (tqm, tqh, tom, tok) and the second auxiliary variable taking into consideration the dependence of the oil flow on the temperature (tqm, tqh, tom, tok), and wherein the non-linear behavior of the oil flow with regard to the pressure drop and flow rate is simulated by a feedback from the oil flow via the hydraulic resistances. The aforementioned aspect, by which the hydraulic resistances in the branches and nodes of the hydraulic network enter in the ascertainment of the vectors of the oil flow in the branches, is also advantageous.
Other aspects of the present invention include, wherein the temperature dependence of the specific resistance of the conductor material is taken into consideration in a return path of the state variables constituted by temperatures to the auxiliary variables constituted by losses. This configuration makes it possible in the calculation of the ohmic and eddy-current losses in a loss-producing part for a given winding current and given leakage flux distribution for the temperature of this part also to be included.
Further aspects include, wherein a feedback of the state variables constituted by temperatures to the hydraulic resistances or to the oil flow is carried out in the thermohydraulic model, the totality of all the oil flows, represented by the vector xcfx86, being temperature-dependent in two ways, namely on the one hand on the driving pressure difference and on the other hand on the viscosity of the oil and consequently the hydraulic resistances. According to this aspect, the temperature dependence of the oil flow is taken into consideration in the thermohydraulic model.
Further aspects of the invention include, an arrangement for carrying out the method, wherein a digital computer is provided and ascertains the thermohydraulic model, with memory units in which the algorithms for the thermohydraulic model are stored, to which computer at least one input keyboard, at least one display device, in particular a screen, and interfaces for preparing the input variables is or are connected. This arrangement has the advantage that it is a common personal computer, in which only the appropriate interfaces have to be installed. The computer should have at least a computer processor (i.e. PENTIUM) with a very high clock frequency.
According to an aspect of the present invention, a method for ascertaining state variables in an oil-cooled transformer is provided. The method includes measuring input variables input variables; establishing a state or status of cooling units affiliated with the transformer; feeding the measurements of the input variables, and state or status of the cooling units into a thermohydraulic model; calculating a present value of state variables in the thermohydraulic model with auxiliary variables, and with a hydraulic network of an oil circuit of the transformer, having a plurality of branches and nodes; calculating a change over time of the state variables; and calculating new state variables based upon the change over time of at least one of the input variables and the status of the cooling units.
According to another aspect of the present invention, the state variables are temperatures. In yet another aspect of the present invention, the state variables include at least one of average temperature of at least one loss-producing part of the transformer, a hotspot temperature in the at least one loss-producing part of the transformer, an average oil temperature of at least one branch of a hydraulic network of the oil circuit, and an average temperature of at least one node of the hydraulic network of the oil circuit.
In another aspect of the present invention, the average temperature in the at least one loss-producing part of the transformer is calculated with the following differential equation:             ⅆ              ⅆ        t              ⁢    tqm    =                    V        tot            -      wim        cqg  
wherein tqm is an average temperature of the least one loss-producing part, Vtot is a total loss of the at least one loss-producing part, cqg is thermal capacity of the at least one loss-producing part, and wim is power loss dissipated from the at least one loss-producing part to oil flowing past the at least one loss-producing part.
In another aspect of the present invention, the power loss wim dissipated to the oil flowing past the at least one loss-producing part is calculated with the formula:   wim  =      V0g    ·                  (                  g          g0                )            xg      
wherein V0g are reference losses, g is a difference between an average temperature of the at least one loss-producing part and an average temperature of oil in contact with the at least one loss-producing part, referred to as average jump, g0 is average jump in a steady state and with the reference losses and xg is an exponent for heat transfer between the at least one loss-producing part and the oil as a function of the oil temperature and oil velocity.
According to a further aspect of the present invention, the hotspot temperature in the at least one loss-producing part of the transformer is calculated with the differential equation:             ⅆ              ⅆ        t              ⁢    tqh    =                    V        hot            -      wih        cqh  
wherein tqh is the hotspot temperature of the at least one loss-producing part, Vhot is a total loss of the at least one loss-producing part multiplied by a xe2x80x9chotspot factorxe2x80x9d, cqh is thermal capacity of the at least one loss-producing part converted to conditions at a location of maximum temperature, and wih is power loss dissipated by a hottest location of the at least one loss-producing part to oil flowing past the at least one loss-producing part.
In another aspect of the present invention, the power loss wih dissipated by the hotspot to the oil flowing past the at least one loss-producing part is calculated with the formula:   wih  =      V0h    ·                  (                  h          h0                )            xh      
wherein V0h are reference losses for the hotspot, h is a difference between a maximum temperature of the at least one loss-producing part and the oil temperature at the hottest location of the at least one loss-producing part, referred to as hotspot jump, h0 is hotspot jump in the steady state and with the reference losses for the hotspot, and xh is an exponent for heat transfer between the at least one loss-producing part and the oil as a function of the oil temperature and oil velocity.
In another aspect of the present invention, the average oil temperature in the at least one branch of the hydraulic network of the oil circuit is calculated with the differential equation:             ⅆ              ⅆ        t              ⁢    tom    =            wim      -      wam      -              phz        ·        D                    coel      z      
wherein tom is an average oil temperature of the at least one oil flow branch, wim is power loss dissipated by the at least one loss-producing part to oil flowing past the at least one loss-producing part, wam is power loss dissipated by the at least one branch to ambient temperature, phz is the oil flow through the at least one branch, expressed in transposed thermal output per degree of difference in temperature between the oil temperature at a beginning of the at least one branch and the oil temperature at an end of the at least one branch, D is a difference in temperature between the oil temperature at the beginning of the at least one branch and the oil temperature at the end of the at least one branch, and coelz is the thermal capacity of the oil in the at least one branch.
According to a still further aspect of the present invention, the thermal power warn dissipated by the at least one branch to ambient temperature is calculated with the formula:   wam  =            wam0      ·                        (                                    υ              xc2x0                                                      υ                0                            xc2x0                                )                          x          ⁢                      xe2x80x83                    ⁢                      υ            xc2x0                              ·      fup        -    VK    -    sun  
wherein wam0 is a reference value of the dissipated thermal power,  is a difference in temperature between average oil temperature in the at least one branch and the ambient temperature, 0 is a reference value for a difference in temperature  for which the value wam0 is defined,  is an exponent for heat transfer between the oil and ambient temperature, as a function of the manner of cooling, fup is a factor for at least one of the influence of the ambient temperature and if appropriate the air pressure, VK is leakage power loss in at least one of a tank of the transformer and a cooler branch which represents a surface of the tank, and sun is the power of solar irradiation on the tank.
Further aspects of the present invention include, wherein the average oil temperature in the at least one node of the hydraulic network of the oil circuit is calculated with the differential equation:             ⅆ              ⅆ        t              ⁢    tok    =                    ∑                  i          =          1                          n          z                    ⁢              xe2x80x83            ⁢              phz        ·                  T          eff                            coel      K      
wherein tok is an average oil temperature in at least one node, nz is a number of branches which enter the at least one node, phz is the oil flow through at least one entry branch, Teff is the temperature of the oil at an end of at least one branch connected to the at least one node, which is dependent on direction of flow, and coelk is thermal capacity of the oil in the at least one node.
According to other aspects of the present invention, for each individual branch in the hydraulic network, a vector for a virtual driving pressure difference f, is defined by the equation:
f=gxc2x7xcfx81xc2x7xcex2xc2x7xcex94Hxc2x7T+pd
wherein f is the driving pressure difference, g is acceleration due to gravity, xcfx81 is the density of the oil, xcex2 is a coefficient of expansion of the oil, xcex94H is a difference in height between a starting point and an end point of the branch, T is an average temperature of the oil in the branch, and pd is a pressure difference caused by a pump in the branch, and wherein a vector xcfx86 of a totality of all the oil flow in the individual branches is calculated with the matrix equation:
xcfx86=TTRTTxc2x7f,
wherein TTRTT defines a matrix which contains information on hydraulic resistances in all the branches and on structural characteristics of the hydraulic network.
Other aspects of the present invention include, ascertaining the vector xcfx86 of the oil flow is in an iterative process which is continued until an initial value and result coincide with a predetermined adequate accuracy. According to another aspect of the present invention, a feedback of the state variables constituted by temperatures of at least one of the hydraulic flow resistances or the oil flow is carried out in the thermohydraulic model, the totality of all the oil flows, represented by the vector xcfx86, being temperature-dependent in two ways, on one hand on a driving pressure difference and on the other hand on the viscosity of the oil and consequently the hydraulic flow resistances.
According to a further aspect of the present invention, when resolving differential equations for the average oil temperature in the at least one branch and in the at least one node of the hydraulic network of the oil circuit, the state of flow is also continuously calculated by a system of equations for pressure drops in all the branches of the entire hydraulic network.
According to a still further aspect of the invention, a continuous adaptation of parameters is carried out with two auxiliary variables, a first auxiliary variable taking into consideration a dependence of the losses on the temperature and a second auxiliary variable taking into consideration a dependence of the oil flow on the temperature, and wherein non-linear behavior of the oil flow with regard to pressure drop and flow rate is simulated by a feedback from the oil flow via the hydraulic resistances.
According to another aspect of the present invention, a temperature dependence of a specific resistance of conductor material is taken into consideration in a return path of the state variables constituted by temperatures to the auxiliary variables constituted by losses. In yet another aspect of the present invention, the input variables comprise at least one of voltages at terminals of the transformer, currents in windings of the transformer, and ambient temperature surrounding the transformer.
In another aspect of the present invention, the voltage measurements are used to ascertain a relationship between the voltages and magnetic flux leakage in individual parts of a core of the transformer by the use of a voltage-magnetic leakage flux assignment matrix, and wherein idling losses are subsequently ascertained in as a function of the magnetic flux leakage by a characteristic curve defined by predetermined parameters.
According to a further aspect of the present invention, internal winding branches of the transformer are defined, linked with the current measurements, by the use of a current assignment matrix, all the current measurements being represented as a vector with two components such that phase position is taken into consideration, and wherein an amount of current with which ohmic losses are calculated, via an appropriate resistance, is ascertained for each internal winding branch, and wherein distribution of magnetic flux leakage is ascertained from a distribution of currents between individual windings with matrices which are defined both for axial components and for radial components, and from the matrices eddy-current losses in the individual windings and in inactive parts of the transformer are ascertained, and wherein a relationship between the currents in the internal winding branches and the magnetic flux leakage relevant for loss-producing branches is ascertained by a currents-internal windings currents matrix, and the eddy-current losses are calculated with factors which are ascertained from winding geometry of the transformer.
In another aspect of the present invention, the voltage-magnetic leakage flux assignment matrix, the currents-internal winding currents matrix, and the ohmic winding resistances are checked and changed if appropriate. According to a still further aspect of the present invention, the matrices, the relevant assignment matrix, and ohmic winding resistances are dependent on the at least one switch position.
Other aspects of the invention include, taking the current measurements at the terminals of the transformer. Further aspects of the invention include, providing a digital computer which ascertains the thermohydraulic model, with memory units in which algorithms for the thermohydraulic model are stored, to which computer comprises at least one input keyboard, at least one display device, and at least one interface for preparing the input variables is connected.
According to other aspects of the present invention, the at least one display device includes a screen. According to another aspect of the present invention, the cooling units include at least one of fans and pumps. According to a still further aspect of the invention, the state or status of the cooling units is included in at least one of (1) in parameters of heat transfer between the oil of the transformer and ambient temperature, and (2) in a corresponding relationship between the cooling units and cooling branches, which is expressed in an assignment matrix.
In yet another aspect of the present invention, a rate of change of the state variables and the state variables is calculated with differential equations. In another aspect of the present invention, the differential equations are resolved by numerical methodology according to Runge-Kutta. According to a further aspect of the present invention, a position of at least one switch of the transformer is fed into the thermohydraulic model as one of the state variables. In another aspect of the present invention, an initializing operation is carried out with every change of the position of the at least one switch. According to another aspect of the present invention, the at least one switch includes a stepping switch.
Other aspects of the present invention include, adapting the auxiliary variables as a function of a present value of the state variables when there is a change in at least one of the input variables, the state or status of the cooling units, and the at least one switch position. Further aspects of the present invention include, wherein the auxiliary variables include at least one of losses in the transformer, parameters for heat transfer, flow resistances and the flow or flow rates in the oil circuit. According to another aspect of the invention, the branches and nodes are assigned a certain oil volume, which together corresponds to a total oil volume.
According to another aspect of the invention, a method for ascertaining state variables, in particular temperatures, in an oil-cooled transformer, is provided. The method includes measuring input variables comprising at least one of voltages at transformer terminals, currents in windings and ambient temperature; defining at least one of a state of cooling units of the transformer comprising at least one of fans and pumps, and a position of at least one switch; feeding the input variables and the status of the cooling units and the position of the at least one switch into a thermohydraulic model; calculating state variables in the thermohydraulic model with auxiliary variables which comprise at least one of losses in the transformer, parameters for heat transfer, flow resistances and flow or flow rates in an oil circuit and a hydraulic network of the oil circuit which has branches, wherein the state variables comprise at least one of average temperatures and hotspot temperatures in loss-producing parts of the transformer, and average oil temperatures in branches and in nodes of the hydraulic network of the oil circuit; adapting the auxiliary variables as a function of the present value of the state variables when there is a change in at least one of the input variables and the status of at least one of the cooling units and the switch position; calculating a change over time of the state variables; and calculating new state variables based upon the change over time of at least one of the input variables, the state of the cooling units, and the position of the at least one switch.
Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawing.